Exceptionally High Carrier Mobility in Hexagonal Diamond

Abstract

Hexagonal diamond (h-diamond), or Lonsdaleite, has been reported to be a wide-bandgap semiconductor with high thermal conductivity and hardness. Our $ab initio$ calculations show that h-diamond has exceptionally high carrier mobility. Along $xy$ and $z$ directions, the hole mobilities at 300 K are 5631 and 5552 cm$^2$V$^{-1}$s$^{-1}$, and the room-temperature electron mobilities are 11462 and 28464 cm$^2$V$^{-1}$s$^{-1}$, respectively. These values are superior to the mobility of most known semiconductors including cubic diamond (c-diamond). The small effective masses in h-diamond, comparable to those in c-diamond, cannot explain the mobility difference between the two phases. For holes, scattering induced by transverse acoustic phonons is the predominant mechanism near room temperature in c-diamond, whereas considerably suppressed in d-diamond by selection rules. The high electron mobility in h-diamond can be attributed to the wavefunction at the conduction band minimum, which is extended and distributed primarily in the lattice interstitial, leading to weak coupling with scattering potentials. The temperature dependence of h-diamond is investigated as well, which deviates from the power-law relationship due to the significantly increased occupation of optical modes at elevated temperatures. Consequently, our findings reveal h-diamond as a promising high-mobility semiconductor, and elucidate the microscopic origin in terms of the carrier-phonon scattering mechanisms beyond conventional understandings based on simple parameters such as effective mass.

Figures (3)

• Figure 1: Carrier mobilities of (a) h-diamond and (b) c-diamond, as functions of temperature. Experimental data are from Ref. Gabrysch2011.
• Figure 2: (a) Phonon dispersion of h-diamond. (b) Calculated $M(\hbar\omega)$ for holes and electrons at 300 K. Two phonon frequency ranges characterized by significant acoustic-phonon scattering and optical-phonon scattering are highlighted with turquoise and violet, respectively. (c)--(j) Zoomed-in views of these two regions, with $M(\hbar\omega)$ evaluated at various temperatures.
• Figure 3: Real-space distributions of charge density at VBM or CBM. The isosurfaces denote the values of (a) $3.5\times10^{-4}$, (b) $1.8\times10^{-4}$, (c) $7\times10^{-4}$, and (d) $1.08\times10^{-3}$ a.u., respectively. Visualized using vestavesta.